ABSENCE OF A PREVALENT LAMINAR DISTRIBUTION OF IPSPS IN ASSOCIATION CORTICAL-NEURONS OF CAT

The depth distribution of inhibitory postsynaptic potentials (IPSPs) w as studied in cat suprasylvian( association) cortex in vivo. Single an d dual simultaneous intracellular recordings from cortical neurons wer e performed in the anterior part of suprasylvian gyrus (area 5). Synap tic responses were obtained by stimulating the suprasylvian cortex, 2- 3 mm anterior to the recording site, as well as the thalamic lateral p osterior (LP) nucleus. Neurons were recorded from layers 2 to 6 and we re classified as regular spiking (RS, a = 132), intrinsically bursting (IB, a = 24), and fast spiking (FS, n = 4). Most IB cells were locate d in deep layers (below 0.7 mm, a = 19), but we also found some IB cel ls more superficially (between 0.2 and 0.5 mm, n = 5). Deeply lying co rticothalamic neurons were identified by their antidromic invasion on thalamic stimulation. Neurons responded with a combination of excitato ry postsynaptic potentials (EPSPs) and IPSPs to both cortical and thal amic stimulation. No consistent relation was found between cell type o r cell depth and the amplitude or duration of the IPSPs. In response t o thalamic stimulation, RS cells had IPSPs of 7.9 +/- 0.9 (SE) mV ampl itude and 88.9 +/- 6.4 ms duration. In IB cells, IPSPs elicited by tha lamic stimulation had 7.4 +/- 1.3 mV amplitude and 84.7 +/- 14.3 ms du ration. The differences between the two (RS and IB) groups were not st atistically significant. Compared with thalamically elicited inhibitor y responses, cortical stimulation evoked IPSPs with higher amplitude ( 12.3 +/- 1.7 mV) and longer duration (117 +/- 17.3 ms) at all depths. Both cortically and thalamically evoked IPSPs were predominantly monop hasic. Injections of Cl- fully reversed thalamically as well as cortic ally evoked IPSPs and revealed additional late synaptic components in response to cortical stimulation. These data show that the amount of f eed forward and feedback inhibition to cat's cortical association cell s is not orderly distributed to distinct layers. Thus local cortical m icrocircuitry goes beyond the simplified structure determined by corti cal layers.